1. Field of the Invention
The present invention relates to an image forming apparatus that forms an image on a moving member.
2. Description of the Related Art
An image forming apparatus having an image forming unit using laser light is known as an image forming apparatus for obtaining a high resolution image. An image forming method of this image forming apparatus includes rotating a drum while using an optical scanning device to scan laser light on the photosensitive drum in an axial direction, thereby forming a latent image on a surface of the drum, and forming a toner image by causing toner to be attracted to the latent image, and transferring the toner image onto paper. In this kind of image forming apparatus, the toner image on the drum needs to be formed at an appropriate position with respect to the paper onto which the toner image is to be transferred.
In particular, in a color image forming apparatus which forms multi-image and color image by overlaying multiple toner images of which colors are different from each other, failure to adjust the toner image of each color to an appropriate position with respect to paper results in various kinds of abnormal image output. The abnormal image output includes, for example, deviation of registration which is deviation of the positions where the color images are written, deviation of magnification rate which causes size error of each color image, and deviation of color due to deviation of positions of multiple toner images relative to each other.
Therefore, when a color image is formed, it is necessary, in particular, to find and adjust appropriately the positions where the toner images are transferred, for each of the toner images for each color. In order to do this, it is necessary to detect the position of the toner image and if it is deviated from the appropriate position, it is necessary to perform control to adjust this, so that the transfer position of the toner image is maintained at the appropriate position.
A known method for appropriately controlling the position of the toner image includes a method for forming a test pattern for position detection on the moving member on which the toner image is formed, and calculating the toner image position by detecting the position of the test pattern. In this method, light is emitted onto the moving member, and calculation is performed with predetermined algorithm using detection information of reflected light from the test pattern, so that the position of the toner image is obtained. Another method is known, in which a test patch for density detection is formed on a moving member, and calculation is performed by predetermined algorithm using detection information of reflected light from the test patch (the amount of received light), so that the density of the image is obtained.
Various kinds of reflection-type optical sensors are known as sensors for receiving reflected light from “the test pattern for the position detection” and “the patch for the density detection”.
For example, an image forming apparatus is known, that uses a reflection-type optical sensor of a type having one light-emitting unit (for example, LED) and one light-receiving unit (for example, PD), and can find the toner image position and the density in accordance with the above image forming method (for example, see Japanese Patent Laid-open No. 2003-241472).
The image forming apparatus of Japanese Patent Laid-open No. 2003-241472 uses a reflection-type optical sensor in order to detect the test pattern for the position detection. The test pattern detection method by the image forming apparatus is as follows. Light emitted by a light-emitting unit provided in a reflection-type optical sensor is radiated upon the test pattern for the position detection formed on a moving member (for example intermediate transfer belt), and light reflected by the portion of the moving member and the light reflected by the portion of the test pattern to which the toner is attached are detected from among the emission light.
Due to scatter and absorption by the toner, the amount of reflected light decreases in the light reflected by the portion of the test pattern. Therefore, the magnitude of an output signal of the light-receiving unit caused by the amount of reflected light in the portion of the moving member is different from the magnitude of an output signal of the light-receiving unit caused by the amount of reflected light in the portion of the test pattern. Accordingly, the test pattern can be detected by determining whether the output signal of the light-receiving unit crosses a reference threshold level (threshold value) or not.
Not only the image forming apparatus of Japanese Patent Laid-open No. 2003-241472 but also an image forming apparatus using a reflection-type optical sensor for detecting toner density required for image formation control is known (for example, see Japanese Patent Laid-open No. 2009-216930). This image forming apparatus detects the test pattern, using one LED as a light-emitting unit and causing a light-receiving unit, made of two photodiodes, to receive the reflected light of the light emitted from the LED onto the test pattern.
The image forming apparatus of Japanese Patent Laid-open No. 2009-216930 uses a reflection-type optical sensor for control of density of an image. However, when light emitted from one LED in the reflection-type optical sensor is radiated upon the patch for the density detection, and the portion of one photodiode (PD) receiving the regular-reflected light is shared, the test pattern for the position detection can be detected, and the position of the image can be controlled on the basis of the detected information. In such reflection-type optical sensor, there is only one light spot on the moving member which illuminates the test pattern, and the size thereof is 2 to 3 mm.
In general, the size of the test pattern for the position detection is 15 mm or more in a direction (main scanning direction) perpendicular to a moving direction (sub-scanning direction) of a moving member on which the test pattern is formed (for example, intermediate transfer belt). In general, the size thereof in the sub-scanning direction is about 1 to 2 mm. On the other hand, the size of the patch for the density detection is 15 mm or more in the main scanning direction, and is also 15 mm or more in the sub-scanning direction.
The size of the test pattern in the main scanning direction is 15 mm or more, which is larger than the size of the light spot in the same direction, so that even if there is relative error in the position of the test pattern and the position of the light spot, the test pattern can be illuminated with the light spot.
The relative position error is considered to include
(1) emission position error in the main scanning direction caused by deviation of the light emission direction due to, e.g., attachment error of the reflection-type optical sensor and attachment error of the light-emitting unit, and
(2) position error in the main scanning direction of the test pattern caused by meandering of, e.g., an intermediate transfer belt and a photosensitive drum, and test pattern formation deviation of the position.
By the way, the toner used to form the test pattern is non-affecting toner that does not affect image formation originally. Therefore, when the size of the test pattern increases, the amount of consumed non-affecting toner increases in proportional thereto, and therefore, this increases running cost relating to image formation.
Accordingly, in order to reduce the amount of consumed non-affecting toner, it is required to reduce the size of the test pattern and the like.
However, there is a limitation in reduction of the test pattern. This is because even if there is a relative error between the position where the test pattern is formed and the position of the spot of the emitted light (light spot), the size of the test pattern needs to be of a certain size or larger in order to enable the above detection. For example, the size of the test pattern in the main scanning direction needs to be larger than the light spot. In this manner, in the image forming apparatus of Japanese Patent Laid-open No. 2003-241472 and Japanese Patent Laid-open No. 2009-216930, the test pattern needs to be formed to have a certain size or larger because of the necessity of ensuring the margin for the position error of the light spot.
An image forming apparatus is known, which solves these problems (for example, see Japanese Patent Laid-open No. 2010-039460). This image forming apparatus uses a reflection-type optical sensor having three or more light-receiving units and three or more light-emitting units, thus capable of detecting an appropriate position even if the test pattern is smaller than a conventional one.
According to the position detection method of the test pattern using a reflection-type optical sensor with the image forming apparatus of Japanese Patent Laid-open No. 2010-039460, an accurate position of the test pattern can be derived from calculation even if the light spot is reduced. More specifically, reflected light from the test pattern is captured by multiple light-receiving units, and the position of the test pattern is calculated from output distribution of the multiple light-receiving units, so that even if the test pattern is small, the accurate position can be detected.
However, even with the position detection of the test pattern using the reflection-type optical sensor with the image forming apparatus described in Japanese Patent Laid-open Nos. 2003-241472, 2009-216930, and 2010-048906, there is the following drawback: when there are, e.g., a scratch or a stain on the moving member such as an intermediate transfer belt, the scratch or stain may be falsely detected as the test pattern. When light is emitted onto the scratch or stain formed on the moving member, the reflected light thereof may be attenuated like the case of the toner image.
An image forming apparatus is known, that solves the above problems (for example, see Japanese Patent Laid-open No. 2010-048906). Even when there is a scratch or stain on the moving member, the image forming apparatus has multiple threshold level set for the amounts of received lights received by multiple reflection-type sensors so as to allow appropriate detection of a pattern for position detection, and the position of the test pattern is detected by determining whether the number of test patterns for the position detection known in advance matches the number of signals crossing a threshold level.
However, there still exists a problem in the position detection of the test pattern described in Japanese Patent Laid-open No. 2010-048906. More specifically, when the attenuation of the reflected light with the test pattern is close to the attenuation of the reflected light due to a scratch or a stain of the moving member, it is impossible to determine whether the scratch or the test pattern has caused a signal crossing the threshold level, and therefore, correct determination cannot be made.
As described above, the image forming apparatus of any one of Japanese Patent Laid-open Nos. 2003-241472, 2009-216930, 2010-039460, and 2010-048906 makes false detection of the position of the test pattern due to a scratch or stain of the moving member, and cannot distinguish the test pattern from the scratch on the basis of the output signal of the light-receiving unit.
A method for making determination on the basis of a time at which the output signal crosses the threshold level, instead of relying on the intensity of the output signal (the magnitude of the amount of received light). However, when the output signal based on one light-receiving unit is greatly different from the time interval calculated from the known test pattern interval, it is impossible to determine whether this is caused by great deviation of the position or a scratch or stain.
When detection error such as false detection occurs, it is necessary to carry out the position detection again, and there is a down time. If false detection of the deviation of the position result is received and image forming condition, image is adjusted, the quality may be reduced.
In view of the above, there is a need to provide an image forming apparatus that can correctly detect a test pattern for position detection and make appropriate adjustment so as not to cause abnormal image such as color deviation even if there is a scratch or a stain on a surface of a moving member.